1. Field of the Invention
The present invention relates in general to the technique of modifying inorganic layered minerals. More particularly, it relates to modified layered clay minerals that are readily exfoliated, polymer/clay composites comprising the modified clay minerals, and a method for its manufacture.
2. Description of the Related Arts
Nanocomposites are a new class of minerals that exhibit ultrafine phase dimensions, typically in the range 1-100 nm. Experimental work on these minerals has generally shown that virtually all types and classes of nanocomposites lead to new and improved properties such as increased stiffness, strength, and heat resistance, and decreased moisture absorption, flammability, and permeability, when compared to their micro- and macrocomposite counterparts. Specifically, commercially available Nylon 6/clay nanocomposite shows that the polymer matrix having layered clay minerals dispersed therein exhibits improved mechanical strength, heat distortion temperature, and impermeability to gas and water.
For the preparation of nanocomposites, it has been disclosed that hydrophilic or hydrophobic swelling agents such as long-chain organic cations, and water-soluble oligomers can be intercalated or absorbed between adjacent silicate layers, to thereby increase the interlayer spacing, so that polymer chains can be included between the silicate layers during polymerization of the polymer matrix. However, the layered silicate according to these methods for the most part can not be exfoliated into individual layers (but can only be swollen), because there is not provided a driving force that can absorb the monomers or oligomers between adjacent silicate layers during the polymerization.
To overcome the above-mentioned problem, the present invention provides a novel method where the layered silicate is intercalated with a catalyst that catalyzes the polymerization of the matrix polymer and thus provides the driving force, such that the silicate layers are readily exfoliated during the polymerization and will be uniformly dispersed in the polymer matrix.
3. Prior Art
There have been numerous attempts to make nanocomposites; see for example; J. Mater. Sci.,31(13), 3589-3596, 1996; Japanese patent Application Laid-Open Nos. 63-215775 and 8-151449; WO 96/35764 and 97/09285; U.S. Pat. Nos. 5,093,439, 5,137,991, 5,164,460, 5,514,734, 5,552,469, 5,578,672, 5,576,257, and 5,616,286. Even though in these methods, layered silicates modified with hydrophilic or hydrophobic agents are more readily exfoliated as compared with non-modified silicates, it is still very difficult to have the silicate layers completely exfoliated and uniformly dispersed in a polymer matrix to give nanoscale structures.
For example, in J. Mater. Sci.,31(13), 3589-3596, 1996, a method is disclosed for making polystyrene nanocomposites wherein montmorillonite which has been surface modified with vinylbenzyltrimethylammonium is mixed with styrene monomer and a suitable organic solvent and then polymerization is effected in the presence of the organic solvent. The interlayer spacing of the montmorillonite contained in the resulting composite is expanded from 0.96 nm to 1.72-2.45 nm. Apparently, the clay minerals could not be exfoliated by soly modified with organic molecules.
WO 96/35764 discloses the reaction of smectite-type clays with certain branched chain quaternary ammonium compounds can produce an organoclay product having superior self-dispersing capability when utilized in grease and ink formulations. The organoclay thus obtained has an interlayer spacing of 2.13-3.23 nm, indicating no exfoliation has taken place.
It should be noted that none of the above cited examples of the related prior art suggest producing nanocomposites from clay minerals that are intercalated with a polymerization catalyst.
An object of the invention is to provide a modified layered clay mineral that is readily exfoliated when admixed with a matrix polymer during the polymerization of the matrix polymer.
Another object of the invention is to provide a polymer/clay composite comprising the above clay mineral and a method for producing the same.
To attain the above objects, the layered clay mineral according to the invention is intercalated with a polymerization catalyst, and then admixed with monomers or oligomers of a matrix polymer to undergo polymerization, such that the silicate layers can be exfoliated during the polymerization and uniformly dispersed in the polymer matrix.
More specifically, the layered clay mineral is intercalated with a catalyst that catalyzes the polymerization of the matrix polymer and thus provides the driving force of absorbing the monomer or oligomers between adjacent silicate layers during the polymerization. In consequence, the probability of the polymerization to take place between the adjacent silicate layers to thereby break the interlayer bonding is increased. Accordingly, the silicate layers are readily exfoliated and will be dispersed individually and uniformly throughout the polymer matrix. Therefore, the mechanical and other properties can be improved to a considerable extent even if only a small amount of the layered silicate is present.
The modified clay mineral of the present invention comprises a layered clay mineral that is intercalated with a polymerization catalyst. In accordance with the present invention, the polymerization catalyst is present in an amount ranging from about 0.05% to 10% by weight, and preferably from about 0.1% to 5% by weight, based on the weight of the layered clay mineral.
The polymer/clay composite of the present invention is prepared by admixing the above-mentioned intercalated clay mineral with monomers or oligomers of a matrix polymer, and polymerizing the monomers or oligomers under the catalysis of the polymerization catalyst. The polymer composite thus prepared comprises a polymer matrix and a layered clay mineral uniformly dispersed therein which is intercalated with the polymerization catalyst. In accordance with the present invention, the intercalated clay mineral is present in an amount ranging from about 0.1% to 30% by weight, and preferably from about 0.5% to 10% by weight, based on the total weight of the polymer composite.
The layered clay mineral used in the present invention is preferably a layered silicate having a cation-exchange capacity ranging from about 7 to 300 meq/100 g. The layered silicate suitable for use herein includes, for example, smectite clay, vermiculite, halloysite, sericite, mica, and the like. Illustrative of suitable smectite clays are montmorillonite, saponite, beidellite, nontronite, and hectorite.
The modified clay mineral of the present invention can be admixed with almost any kind of thermoplastic or thermosetting polymers by way of melt blending or oligomer intercalating, followed by polymerization to form polymer/clay nanocomposites. If necessary, oligomers can be first included between the adjacent silicate layers before subjected to polymerization, which results in a better dispersibility of the exfoliated silicate layers in the polymer matrix. The matrix polymer suitable for use in the present invention includes, for example; conductive polymers such as polyaniline, polypyrrole, polythiphene; polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate (PC); silicones such as polydimethyl siloxane, silicone rubber, silicone resin; acrylic resins such as polymethylmethacrylate, polyacrylate; epoxy resins such as bisphenol-epoxy, phenolic-epoxy; and styrene polymers such as polystyrene, styrene-acrylonitrile copolymer, acrylonitrile-butadiene-styrene copolymer.
No particular restrictions are placed on the polymerization catalyst to be used in the invention. They are chosen based on the matrix polymer to be polymerized. The only requirement is that the catalyst must be insensitive to water. The catalyst suitable for use in the present invention includes, for example, antimony acetate that catalyzes the polymerization of PET, 1,8-diazabicyclo[5,4,0] undec-7-ene (DBU) that catalyzes the polymerization of epoxy resins, and cocamidopropylhydroxysultaine that catalyzes the polymerization of polyaniline.
Optionally, the layered clay mineral can be further intercalated with a modifier in addition to the polymerization catalyst. The modifier used herein further expands the interlayer spacing between adjacent silicate layers and to functionalize the clay mineral. The modifier suitable for use in the present invention has a functional group that is reactive with the polymer matrix, including such as carboxyl, hydroxyl, carbonyl, vinyl, sulfonyl, and epoxy groups. Preferably, the modifier is an amido compound. The modifier intercalated at the clay mineral will react to and therefore bond to the polymer matrix to thereby improve mechanical reinforcement, or to increase heat resistance and decrease water permeability. The modifier suitable for use in the present invention includes, for example; commonly used surfactants such as cocamidopropylhydroxysultaine, cocoamphoropionate, and cocoamphoacetate; coupling agents such as glycidyl phthalimide, pentaerythritol polygiycidyl ether, and phenyl glycidyl ether; and compatilizers such as (MeO)3Si(CH2)3SH, and (EtO)3Si (CH2)3NH2. In accordance with the present invention, the modifier is present in an amount ranging from about 0.05% to 10% by weight, and preferably from about 0.1% to 5% by weight, based on the weight of the clay mineral.
The polymer/clay composite of the present invention may be further incorporated with additives such as organic or inorganic fillers, antioxidants, UV light absorbers, light stabilizer, antistatic agents, flame retardants, and lubricants according to the intended use.
Without intending to limit it in any manner, the present invention will be further illustrated by the following examples.